Insulating bushings and method of making the same



Nov. 13, 1928.

ML MORRISON INSULATING BUSHINGS AND METHOD OF MAKING SAME Filed Sept. 13, 1925 INVENTQR Momfom Momson.

WITNESSES:

ATTQRNEY Patented Nov. 13, 1928.

UNITED STATES HONTIORD MORRISON, OF CHICAGO, ILLINOIS.

INSULATING BUSHINGS AND METHOD OF MAKING THE SAME.

Application filed September 13, 1823. Serial No 662,458.

My invention relates to insulating bushings and to methods of, making the same and particularly to. structures and methods of making insulating bushings of the condenser t e.

lsne object of my invention is to provide a method of making insulating bushings that shall facilitate the manufacture thereof, as regards bot-h speed and economy.

Another object of my invention is'to provide a method of so making insulating bushings from preformed-stock insulating tubes that they may be produced in any desired sizes, on short notice and in comparatively short eriods of time. I

A urther object of m invention is to provide an insulatin bus ing that shall be simple and durable in construction and effective in its operation.

Heretofore, while various methods have been suggested for manufacturing insulating bushings of the condenser typle, all of them,

to the est of my knowledge, ave been relatively complicated, and therefore, expensive.

As before stated, w ile various methods have been suggested, such as placing concentric cylinders in relatively loose concentric or telescopic relation and pouring liquid material between the cylinders, the method most generally adopted, for providing a compact and effective bushing, has been to successively wind alternate layers of insulating sheet material and metal foil on a single mandrel.

The first of the above mentioned methods is not only cumbersome and expensive, but is likely to result in a defective bushing, if bubbles or weak spots occur in the liquid, whether the latter be liquid metal poured between insulating cylinders 0r 1i uid insulation poured between metal cylin ers.

The second method, that of winding sheet insulating. material and metal foil on a single mandrel, requires considerable time and sklll, in addition to requiring that various sizes of complete bushings be kept in stock for use on short notice.

This method is also subject to the objection that, if a wrinkle, misplacement or other defect occurs during the winding operation, the accumulative effect of the fault will usually be reater in accordance with the thickness 0 the material wound over the defect. A bushing made of a solid homogeneous mass of insulating material, such as may be formed by pouring a liquid material 'tion as to cause brea into a mold and allowing it to harden, is very likely' to contain air pockets, fissures, minute differences of density or composition, or other features severall or in such combinalidown under voltage stress. Thus, solid bushings of the same size, shape and material may operate difl'erently under the same voltage conditions.

It is m aim to overcome the above-mentioned ob ections by providin a method by which bushings may be so made from standard stock parts as to facilitate manufacturing operations, promote economy and ensure a product thatshall be effective in operation. Thus, in building up a bushing comprisin a number of such cylinders, a defective cy inder may be detected and rejected before it is embodied in the bushing and other advantages obtained that are not incident to the continuous winding or molding pro- CGSSBS.

In practicing my invention, I provide a. method by which preformed standard stock tubes or cylinders are so placed in concentric relation, under tension or compression forces between the tubes and between thetubes and an internal conducting lead or terminal, as to facilitate speed and economy 'of manufacture and to ensure durability and effectiveness'in operation.

The method may com rise the endwise forcing of preformed cy inders into tight fitting or tensioned concentric relation, or, preferably, the successive heating and shrinking of the cylinders one upon another.

igure 1 of the accompanying drawings, is a view, partially in side elevation and partially in longitudinal section, of a condenserterminal bushing embodying, and constructed in accordance with the method of my invention, and

Fig.2 is a plan view of the bushing shown in Fig. 1.

The bushing l comprises insulating cylinders or tubes 2 of different lengths and diameters disposed in concentric or telescopic relation to each other and to a conductor 3, with relatively thin metal tubes 4 therebetween.

If, as suggested in the prior art, either-the .insulating cylinders 2 or the metal tubes 4 be preformed and the others be poured into position. as a liquid, the operatlon will be cumbersome. Also, by reason of the thinness of the walls of the various cylinders and the uniformity of shape and texture required the pouring method is not likely to be successful on a commercial basis, as, in fact, it has not been.

In one form of my invention, theinsulating tubes 2 are initially separately and independently formed, as by a usual method of winding sheets of paper impregnated with a binder. The tubes may be wound to any desired len th and afterwards cut to suit the individual bushings for which they are intended.

Being wound on separate mandrels, the inner surfaces of the several tubes are of uniform diameter and contour from end to end.

Before or after being cut to proper lengths, as illustrated in Fig. 1, each of the cylinders, or each except the outer cylinder, may be provided with a thin outer coating of conducting material, such as metal foil held in position by shellac or other substance. The cylinder of smallest diameter may then be heated to enlarge its inner diameter by expansion, and then slipped over the conductor 1 and allowed to shrink tightly into position thereon.

Since the cylinders are of insulating material, the inner surface of each, as successively heated and cooled, will better adapt or accommodate itself to minor irregularities in the outer surface of the next preceding cylinder than is the case where metal members are so shrunk one on another. In other words, by reason of the relative pliability of the insulating cylinders, as compared to the stiffness of metal members, microscopic irregularities may be better compensated for by having the above-mentioned inner and outer surfaces in intimate point-to-point contact throughout the entire areas thereof.

,Such intimate contact is usually not required of metal members joined for mechanical purposes but is important in condenser bushings where high electrostatic stresses are involved.

The above operation may, of course, be omitted and the first operation be the heating of the second cylinder and allowing the same to shrink into osition over' the first cylinder without having the conductor 3 in position. In either case, the remaining cylinders may be similarly placed in position, to any desired number.

A modification of my method is to force the insulating cylinders 2 endwise over the conductor 3 and each other, the cylinders being, in each form, of such diameters when cool, or of'the same normal tem erature, as to require either heating and s rinking or longitudinal force to permit the assembly thereof.

Another modification of my invention is to place previously prepared cylinders, in concentric relation, and forcing shellac or other material therebetween in a vacuum. In each of the above described forms, the

cylinders are caused to set in position in the bushin under tension, with compression forces t erebetween, in such manner that, to the best of my knowledge, has never been emplo ed before.

Bus ings made up of a plurality of preformed stock cylinders, in the manner of my invention, have other important advantages which tend toward the ready economical production thereof and to render the same more effective than continuously-wound bushings, solid-material bushings or concentric-preformed-cylinder bushings that may be placed together by ordinary methods, as by placing the cylinders relatively loosely together and pouring a liquid therebetween.

In a solid-material bushing, an inner fissure, a weakened area or stratum or other defect may be caused to increase under voltage. stress, changing temperatures and atmospheric conditions or with the effects of age. In the laminated structure of my invention, such a defect may be confined to a v local spot between the adjacent cylinders.

A continuously-wound bushing, as usually made from one stock of rolled paper, may be uniformly good or uniformly. poor in accordance with minor variations in the texture and composition of the paper, the bushing being made up entirely of one stock or supply of paper. Thus, continuously-wound bushings of the same size and of ap arently the same material may differ in c aracter because of differences occurring between successive supplies of the same paper.

By my invention, whereby the cylinders employed in the bushings may be made up in stocks of large numbers of each size, the random selection of the cylinders from such stocks is believed to be more conducive to the production of bushings of uniform character. In other words, the cylinders in each bushin may severally be of different degrees o excellence of the same general quality or class of paper, so that an average quality in the bushings may be more nearly obtained without the care and expense necessary to ensure a similar average in bushings of the other types mentioned. It is also thought that the exposure of the tubes to the atmosphere and other elements, temperature variations, chemical chan es or readjustments and other effects, while aging in stock ready for use, together with the heating and cooling process during the assembly of a bushing, may modify the surface characteristics of the cylinders sufliciently to more nearly equalize the stress distribution through the layer sections.

It has been demonstrated that the treatment of the tubes, whether the treatment include the application ofmetal foil thereto or not causes the bushings to operate with desirable condenser bushing characteristics and, in certain service, to perform, without the ill) foil, much better than solid-material bushings and as satisfactory substitutes for bushings in which the foil is employed. In the latter instance, economy of materials and labor are obtained to a marked degree over any bushing of the prior art of which I am aware.

My invention also comprehends the obvious operation of reversing the order in which the cylinders are assembled, that is, in initially placing the next to'the largest cylinder within the largest cylinder and so on.

\Vhile I have shown and described particular forms of my invention, changes may be effected therein without departing from the spirit and scope thereof, as set forth in the appended claims.

I claim as my invention:

1. The method of constructing a bushing which comprises placing a heated insulating tube around a conductor and allowing the tube to shrink into position on the conductor.

2. The method of constructing a bushing which comprises placing a heated insulating tube around a conductor, allowing the tube to shrink into position on the conductor and similarly placing one or more heated tubes, and allowing the same to shrink, over thefirst tube after the preceding tube has cooled.

3. The method of constructing a bushing which comprises initially independently constructing a plurality of fibrous insulating cylinders of different diameters, coating a surface of one of said cylinders with a metal body and placing the cylinders together in concentric relation with said metal body in compression therebetween.

4. The method of constructing ,a bushing which comprises initially independently constructing a plurality of fibrous insulating cylinders of different diameters, placing a body of metal foil thereon and placing the cylinders together with the body of metal foil in compression therebetween.

5. The method of constructing a bushing which comprises initially independentl constructing a plurality of fibrous insu ating cylinders of different diameters, coating a r surface of one of said cylinders with a metal body and placing another of the cylinders While heated in concentric relation over the first cylinder to shrink into position thereon.

6. An insulating bushing comprising a current-carrying conductor and a preformed insulating tube surrounding the same under tension.

7. An insulating bushing comprising a plurality of alternately disposed conducting and preformed insulating tubes arranged in concentric relation, each insulating tube being under separate tension over the next inner tube.

8. An insulating bushing comprising a plurality of preformed insulating tubes disposed in concentric relation, each insulating tube being under separate tension, and a body of metal foil on one of said tubes within another thereof.

9. An insulating bushing comprising a plurality of relatively thin walled preformed fibrous insulating tubes disposed in telescopic relation and each under separate tension over the next inner tube.

10. An insulating bushing comprising a plurality of relatively thin walled preformed insulating tubes of organic insulating material disposed in telescopic relation and each under separate tension over the next inner tube.

In testimony whereof, I have hereunto subscribed my name this 8th day of September, 1923.

MONTFORD MORRISON. 

